Crazy oversteer / wheelspin with Michelin Pilot Super Sport

[Ubersuber]

Originally Posted by 7thgear

high as possible before over inflation....

Dont follow this advice. The center of the tread will wear out long before the outside ribs.

ALWAYS follow the manufacturer's recommended pressure or no more than about 3% higher. If your ambient temperature varies a lot during the day then a bit higher is recommended if you check your tires when the ambient is higher than average.

Nearly 1M km and I have never had to replace a tire due to uneven wear. That's a lot of tires all worn perfectly.

[Ubersuber]

Originally Posted by reardrv

Huh, that's strange. Because on my actual car that I drove to work only about an hour ago it reduces the tendency to oversteer.

No it didn't. Lowering rear tire pressure below recommended will reduce understeer. It cannot reduce oversteer, physically impossible.

[Ubersuber]

Originally Posted by stonenewt

Your car weights 3000lbs, you're tyres are at 30psi, it requires 100 sq" of area to support that weight, or 25 sq" per tyre. On a 215 wide tyre that's something around 3.75" of tread length on the road assuming the contact patch is close to an ellipse. For this reason even on very large radius changes or tyre width changes you shouldn't see much increase of contact area.

You are missing the volume effect. There is more air in the larger tire, i.e. the 215/45x17 tire has a smaller air chamber than the 225/45x17.

It is the volume of air that supports the load (technically the mass of the air). The higher the weight the more volume is required or more pressure. The mass of air is required to support the weight of the vehicle.

This isn't rocket science you know. Take all the air out of the tire and see what happens, then add back until the car is supported on the correct contact patch profile. You'll add the same amount of air to each tire but the pressure will have to be higher in the smaller volume tire, which it will be if the contact patch is to be the correct shape. Boyle's Law.

Tire construction affects this but not nearly as much as you might think. Run flat tires exploit the carcass rigidity in order to hold their shape without air but they need the air to work correctly. Hence the invention of TPMS, for run flats.

[Ubersuber]

As for Michelin not knowing or not telling you how to inflate your tires, not so:

http://www.tirerack.com/tires/tirete...nd_Feeding.pdf

Note that this advice is very specific and relates to their streetable competition tire.

For street use they would have a recommended cold tire pressure.

Note the difference between track pressures and auto cross pressures. Street use would require even higher cold pressures.

Note the recommended differential between front and rear pressures, this will reflect the greater heating of front tires on all road cars, even on the track or autocross.

If the rear tires are overheating before the front tires you need to correct that.

[wparsons]

Originally Posted by Ubersuber

Dont follow this advice. The center of the tread will wear out long before the outside ribs.

ALWAYS follow the manufacturer's recommended pressure or no more than about 3% higher. If your ambient temperature varies a lot during the day then a bit higher is recommended if you check your tires when the ambient is higher than average.

Did you even read the post you quoted? Over inflation is when you're inflated to the point that the outer edges aren't being used as much as the center. No one is suggesting running 45psi cold for daily driving.

Originally Posted by Ubersuber

Nearly 1M km and I have never had to replace a tire due to uneven wear. That's a lot of tires all worn perfectly.

Do you want a cookie?

Originally Posted by Ubersuber

No it didn't. Lowering rear tire pressure below recommended will reduce understeer. It cannot reduce oversteer, physically impossible.

How is it physically impossible?

Given that the front carries more weight than the rear, and the tires are the same at all four corners the fronts should run more pressure. Running the same pressure all around is probably running too much in the rear, so dropping it will put it in to a better range.

Again, no one is suggesting dropping it to 25psi cold with 35psi front... we're talking about a couple psi difference.

You should REALLY get out into the real world and experiment with things before making all the incorrect blanket statements that you love so much.

[Ubersuber]

Originally Posted by stonenewt

Non I didn't lie, I simply summarised what they said.


There's one thing in that, non-exhaustive, list they can't actually quantify now. The exact properties of the raw materials used to create the tyres in question. This is what makes it a difficult thing to do. In the extreme when racing at the level that every 10th of a second counts sometimes a particular set of tyres requires a marginally different suspension setup!

The question becomes how close do you want to be to the ideal pressure - 3psi? 1psi? .25psi?

I didn't say you were. If Michelin said they couldn't advise you then they were lying since they have actually published such advice:

http://www.tirerack.com/tires/tirete...nd_Feeding.pdf

The challenge for "correct" tire pressures for street use is the tires do not heat up very much. The recommended cold pressure assumes that ambient temperatures will not vary much from the temperature at which cold pressure is measured and that driving conditions will not overheat the tires. These are valid assumptions.

The owner must allow for the expected changes in ambient temperatures. The best way to do this is to check tire pressures in the morning before the day warms up and set them to recommended at that time. Any other time of day you should set them a little higher depending on what you think the lowest ambient temperature is likely to be. In Canadian winters and checking "cold" tire pressures in a warm garage you have to add a lot of extra pressure to ensure the tire will remain the correct shape when driving outdoors.

[Ubersuber]

Originally Posted by AsianStyle

I have 235/45/17 MPSS on 17x8 wheels, what should the tire pressure be for standard street use be if I want to optimizing tread wear over maximum grip?

35 psi should work fine but you could probably go as low as 33 psi. The higher pressure will get you better grip most of the time but the lower pressure will likely give you more even tire wear.

215/45x17 recommended pressure is 35 psi.

[Ubersuber]

Originally Posted by wparsons

Did you even read the post you quoted? Over inflation is when you're inflated to the point that the outer edges aren't being used as much as the center. No one is suggesting running 45psi cold for daily driving.



Do you want a cookie?



How is it physically impossible?

Given that the front carries more weight than the rear, and the tires are the same at all four corners the fronts should run more pressure. Running the same pressure all around is probably running too much in the rear, so dropping it will put it in to a better range.

Again, no one is suggesting dropping it to 25psi cold with 35psi front... we're talking about a couple psi difference.

You should REALLY get out into the real world and experiment with things before making all the incorrect blanket statements that you love so much.

I don't think you are open to reasonable discussion.

Find one technical source that says reducing rear tire pressure from factory specification will reduce oversteer. It isn't possible.

[Ubersuber]

Originally Posted by stonenewt

Unless you're on a really smooth surface then with tyres under 35mm you actually want to run the lowest pressure you can without the risk of pinch punctures. This includes when you're lent over for long downhill sweeper at say 40mph, which gives less deflection scope than when upright.

As for strength, think about this. If I span a 20cm gap with a straw that can only just support 50g in the middle what happens when I make that gap 2m? The straw collapses. So for the same loading the bike tyre doesn't need to be nearly as strong to start with.

With this in mind let's take a 19mm track tyre with a maximum pressure of 220psi, its bead to bead tension will be about 380lbs. On a 215/45 tyre it would be about 3540lbs! At 45psi, the maximum pressure for the tyres on my car, the tension is around 725lbs. For a 2.5" bike tyre, a typical size for a maximum of 45psi, the tension is about 260lbs.

So basically you can't really compare bike tyre & car tyre pressures.

????

This is dangerously incorrect.

As far as load carrying goes you absolutely can compare bike tires to car tires. The air that goes in them is the same.

No bike carries anything like the loading of a car tire but the same larger volume requires lower pressure to carry the same weight applies to both.

Boyle's Law is a law of nature and does not vary in human experience.

Just one reference ought to be enough:

http://www.schwalbetires.com/tech_in...ation_pressure

Find a technical site that supports the claim that tire pressure does not need to vary with internal tire volume when required to support a given weight.

I mean why do heavy cars use larger tires but roughly the same tire pressure?

Why do you have to add air if you fully load a car?

[Ubersuber]

Originally Posted by troek

this has greatly diverged from the topic, but im enjoying learning about this. what makes taller tires have higher load ratings? IE a 205/55r16 vs a 205/65r16.

More air inside automatically increases the load capacity of a tire but that isn't the reason.

Load ratings are decided upon by tire manufacturers to provide fitment solutions for the vehicles they intend to use those tires.

Tires of a given size usually come in a variety of load ratings and it is very important that you do not fit an inadequate tire to your car even if it is of the correct size.

Furthermore, in addition to plain jane load ratings there is the XL specification which indicates that the tire is stronger than the normal for its size. These tires ride harder but are more resistant to damage. You wouldn't want an XL tire on your BRZ except for rugged winter conditions.

And here's an explanation from someone who claims to know what he's talking about. He appears to:

http://www.barrystiretech.com/loadtables.html

[wparsons]

Originally Posted by Ubersuber

You are missing the volume effect. There is more air in the larger tire, i.e. the 215/45x17 tire has a smaller air chamber than the 225/45x17.

It is the volume of air that supports the load (technically the mass of the air). The higher the weight the more volume is required or more pressure. The mass of air is required to support the weight of the vehicle.

This isn't rocket science you know. Take all the air out of the tire and see what happens, then add back until the car is supported on the correct contact patch profile. You'll add the same amount of air to each tire but the pressure will have to be higher in the smaller volume tire, which it will be if the contact patch is to be the correct shape.

You just don't get it. Air volume doesn't support anything, it's the pressure. Even at 0 psi there is still air inside the tire, but it's not going to support anything.

Pressure required has absolutely NOTHING to do with volume inside the tire, and everything to do with contact patch size. If you want to calculate how much pressure is required, look at the desired contact patch size in square inches, and how much weight each wheel needs to support. The tire pressure will be roughly the weight divided by the contact patch size. This even supports your no pressure situation, in which the contact patch keeps flattening/expanding until the rim hits the ground.

Originally Posted by Ubersuber

As for Michelin not knowing or not telling you how to inflate your tires, not so:

http://www.tirerack.com/tires/tirete...nd_Feeding.pdf

@stonenewt said they didn't have a suggestion for what the pressure should be for his specific car, which if you read that article it even tells you to look at your owners manual. The tire manufacturer won't give you an OEM answer, and if it's not the stock tires the car manufacturer won't give you an answer either because they haven't tested those tires.

You should also REALLY take a close look at the table in that pdf that tells you what affect tire pressure has on handling. I'll give you a hint, it states that decreasing rear tire pressure will decrease oversteer. So much for it being physically impossible.

Originally Posted by Ubersuber

Note the difference between track pressures and auto cross pressures.

You HAVE to be trolling, right? No one has this poor of reading comprehension. The quote below is also taken from the article you linked to.

Originally Posted by MichelinArticle

note: Though autocross hot target pressures are the same
as those for road racing, you may need to start at a higher
cold inflation pressure to compensate for the lower pressure
gains in autocross racing.

Originally Posted by Ubersuber

Street use would require even higher cold pressures.

I bet you get more heat in your tires in daily commuting/driving (not even back road spirited driving) than you'll see in an autox run. It's REALLY hard to get heat into tires in a 35-60 second run.

[Dipstik-sportech]

Where's my godamn popcorn!!!!!

[wparsons]

Originally Posted by Ubersuber

Boyle's Law is a law of nature and does not vary in human experience.

Boyle's law has NOTHING to do with this. All Boyle's law says if that if you have a constant volume of air and reduce the size of the container the pressure will go up and if you increase the size of the container the pressure will go down. It says NOTHING about needing less pressure in a tire to provide the same support.

Originally Posted by Ubersuber

I mean why do heavy cars use larger tires but roughly the same tire pressure?

Because they have tires with significantly larger contact patches. Again, zero to do with volume, everything to do with contact patch size and weight on each wheel.

Originally Posted by Ubersuber

Why do you have to add air if you fully load a car?

How is making a car heavier affecting volume of the tire? You have to add more pressure because you have increased the weight on the wheels without changing desired contact patch size. This means more pressure required.

[wparsons]

Originally Posted by Dipstik-sportech

Where's my godamn popcorn!!!!!

I think he's going to shut up for a while after he posted an article that directly contradicts at least three things he has been saying.